Process for separating tin from lead



Patented Mar. 18, 1941 m'rso. STATES PATENT QFFICE- No Drawing. Application October 23, 1939,

Serial No. 300,801

3 Claims.

This invention relates to lead recovery. Metallic lead derived from various sources, for instance, lead recovered from waste lead products, frequently contains substantial quantities of tin. In some instances, the tin content may be only one or a few per cent, but in lead recovered from waste lead products the tin percentage at times runs all the way up to 20% or even higher. For various reasons it is desirable to remove all or substantially all. of the tin frequently found alloyed with lead.

' In addition to the tin, lead is frequently found combined with other metals, antimony being commonly alloyed with the lead to increase the hardness, and also with others which may be present more or less as impurities, such as copper, silver and bismuth.

In the practice of the prior art, tin was removed from lead by a variety of different processes, such as by oxidation of a molten bath of the alloy of tin and lead. The prior processes,

however, were not economical or efilcient, and commonly resulted in removal of large quantities of lead with the tin. Prior processes for removing tin also frequently resulted in removal of substantial quantities of other metals which might be present, such as those mentioned above, which is undesirable since it is important to maintain a. high degree of purity in the tin removed.

One of the primary objects of the present invention is the provision of a highly effective process according to which tin is separated from lead. The process, moreover, is of advantage since the tin derived from the process as a byproduct is of high purity.

In accordance with another aspect of the in- .vention, the tin separation is accomplished in a manner so that only relatively small amounts of 40 lead are removed with the tin, this being of advantage not only from the standpoint of retaining as much lead as possible in the batch under treatment, but also from the standpoint of maintaining a high degree of purity in the tin removed, the tin also being valuable for various purposes, particularly when it is not combined with large quantities of lead,

Still further, in accordance with the present invention the tin removal is effected without ap- 50 preciable separation of other metals which might be present, such as antimony, copper, silver,- bismuth, etc. This is of advantage since a high degree of purity in the tin removed is desirable.

Briefly, the. method of the present invention involves heating a batch of lead-tin alloy and introducing lead chloride into the molten mass. The lead chloride establishes a condition in which preferential oxidation of the tin takes place, the lead being oxidized only to a very limited degree. The condition established by the lead chloride also 5 results in the separation of very little, if any, of the other metals which might be present. At the end ofthe reaction period the tin oxide and lead chloride are removed from the surface of the mass in the form of a scum. 10

As to tin removal, the present process is effective for the purposes contemplated when maintaining the following conditions of treatment.

The temperature of the molten bath must be above that necessary to melt the lead chloride introduced and preferably above about 1050 F., for the reason that below this temperature the tin oxide formed during the process will not readily separate from the mass. The temperature, moreover, should be kept below about 1200 F., since above this figure the reaction is not as preferential with respect to tin, and the quantity of lead oxidized and removed with the tin increases.

The molten bath, containing the lead chloride, is vigorously agitated, ,as by means of an impeller or the like, adapted to cause a circulating movement of the entire mass in the pot.

The effect of the foregoing is to constantly circulate different portions of the batch to the surface for exposure to the air, and thereby to subject all parts of the mass to the oxidizing influence of the air. If desired, oxidation may be augmented by the use of air jets delivering air into the interior of the mass. Oxidation moreover may be effected even in the absence of air, as by introducing any other suitable oxidizing agent and intimately intermingling the same with all parts of the mass. I

With respect to agitation, particularly where surface oxidation is relied upon, other means for effecting agitation may be. employed, such as paddles, or air jets, although I prefer an impeller, as above mentioned, since I have found the vortical type of circulation set upto be highly effective.

The lead chloride may be introduced in a number of different ways. In the first place, it may be introduced directly, as by adding lead chloride to the mass; Alternatively, the lead chloride may be introduced by adding a chloridizing agent, such as ammonium chloride, antimony chloride or cuprous chloride, or by blowing chlorine gas through the mass. Any of these latter methods will result in the production of lead chloride by reaction in the mass.

As to the quantity of lead chloride present, it may be noted that virtually any amount above a trace will produce some efiect in setting up a preferential oxidation, although, naturally, where substantial quantities of tin are to be removed, appreciable amounts of lead chloride should be employed, as noted, for instance, in the examples given herebelow. For most practical purposes, the amount of lead chloride present in the batch should be at least 2% of the tin by weight.

For various reasons I prefer to use a quantity of lead chloride not greater than about 10% by Weight of the tin contained in the batch. While for some purposes the quantity used may be a little higher, in most instances it will be found that substantially less than the percentage of lead chloride just mentioned will effectively carry out the reaction. Furthermore, since lead chloride is removed with the oxide scum at the end of the treatment period, in accordance with the preferred practice of my invention the amount of lead chloride is kept sufiiciently low so that the tin oxide produced may be worked directly without any previous removal of lead chloride. Separation of lead chloride from tin oxide is a diincult and costly procedure.

The scum is removed from the surface of the mass after the agitation period but before cooling. In large part the scum consists of oxides, and with the process of this invention I have found that the metallic tin content of such removed scum is at times as high as 60% of the total removed materials.

During oxidation some lead oxide is formed. In fact, the presence of lead oxide is apparently necessary for successful carrying out of the reaction, and I have found that initial introduction of some lead oxide with the lead chloride aids in initiating the reaction. I, therefore, prefer to introduce both lead oxide and lead ch10- ri'de together, although it may be noted that the quantity of lead oxide introduced is not critical. For most purposes, introduction of a mixture of lead chloride and lead oxide containing from about 25% to about 50% of the oxide serves the purpose.

In connection with the matters of agitation and oxidation of the batch under treatment, it

should be noted that the time of treatment depends principally on the rate of agitation and the efiiciency of the oxidizing agent. The im- Dortant consideration is to so handle the material under treatment as to bring the oxidizing agent into intimate contact with substantially all parts of the batch throughout the mass'thereof. When relying for oxidation on the air at the surface of the batch, the intimate contact is efi'ected by vigorous agitation as by means of a circulating impeller, the result 01' the circulation being to bring all parts of the mass to the surface for oxidation.

Where reference is made in the appended claims to subjecting the batch to an oxidizing agent throughout the mass thereof, it is contemplated that all parts of the mass may either be circulated to the surface for oxidation or may be brought in contact with the oxidizing agent, as by blowing air or stirring some other agent within the body of the batchl Example 1 of iinseparation A batch of 100,000 ass. or lead containing -1.25% tin and 4 antimony was placed in a pot and heated to 1120 F. 200 lbs. of a mixture of lead chloride and lead oxide (100 lbs. of each) was introduced into the molten mass and agitation by means of an impeller of the type above referred to was carried on for about five hours. 6 The oxide scum removed contained metallic tin and 2% antimony. The lead remaining in the pot contained only 15% tin.

Example 2 of tin separation A batch of 50,000 lbs. was introduced into a pot, the same containing 4% tin, 25% antimony, .5% copper, and the balance lead. Treatment was efiected by introducing 200 lbs. of a mixture of lead chloride and lead oxide (about 150 lbs. of the chloride and lbs. of the oxide), the mass being agitated for about five hours at 1080 F. The resultant scum at the end of the treatment period contained metallic tin and 4% antimony.

The treatment above was repeated twice, after which the lead remaining in the pot contained 25% tin and the percentage of antimony had risen slightly above the original 25% figure.

In instances where the batch being treated 25 contains certain impurities, it may be found that the lead chloride is used up or spent by combining with such impurities, and in this event more chloride should be added during the process.

In carrying out the process, following the treatment period, the scum is removed and allowed to cool, after which the scum is screened so as to remove lead particles or shot which are picked up from the batch during agitation and held in the scum mechanically. These lead particles may, of course, be recombined with'any other lead batches.

After screening, the oxide mass may be smelted, preferably after briquetting so as to avoid dust losses in the smelting process. It should be noted in this connection that when properly controlling the quantity of lead chloride employed, this working of the tin oxide may be effected without previous treatment to remove lead chloride. The alloy produced is very high in tin content and thus is suitable for producing solder. Furthermore, it will be found that even where the tin is separated from an initial batch containing other metals and impurities, the tin is relatively pure, containing only very small amounts, if any, of the other metals. If necessary or desirable, the metallic tin produced by the smelting may be refined to remove other ingredients, such as antimony.

The foregoing portion of the process, therefore, provides for effective separation of tin from lead, thereby producing lead of very low tin content and at the same time producing a tin alloy of high tin contentsuch as is suitable for use as a, solder base.

I claim:

1. A-process for separating tin from lead comprising subjecting a batch of lead and tin to an oxidizing agent throughout the mass thereof in the presence of lead chloride and .at a temperature sufficient to melt the lead chloride but not above about 1200 F., the quantity of lead chloride being from a trace to about 10% of the weight of the tin in the batch, and removing the resultant powdery oxide mass from the surface of the batch.

2. A process for separating tin from lead comprising adding lead oxide to a batch of lead and tin, and subjecting the batch to an oxidizing agent throughout the mass thereof in the presence of lead chloride and at a temperature sufilcient to melt the lead chloride but not above about 1200 F., the quantity of lead chloride being from a trace to about 10% of the weight of the tin in the batch, and removing the resultant powdery oxide mass from the surface of the batch.

3. A process for recovering metallic tin from lead-tin alloys comprising subjecting a batch of lead and tin to anoxidizing agent throughout the mass thereof in the presence of lead. chloride and at a temperature sufiicient to melt the lead chloride but not above about 1200 F., the quantity of lead chloride being from a trace to about 10% of the weight of the tin in the batch, removing the resultant powdery oxide mass from the surface of the batch, and, without intermediate treatment beyond mechanical conditioning, subjecting said oxide mass to a smelting operation to substantially directly reduce the oxide to metallic tin.

ROBERT B. ERICKSON. 

